84- METALLURGY OF IRON AND STEEL.
part may exist in combination as cementite, and part as graphite. Steel containing 0.90 per cent, of carbon, if cooled slowly, will be mostly pearlite., but will usually contain a trace of graphite and some cementite. Pig-iron with 4 per cent, of carbon cannot contain more pearlite than the steel just mentioned, but there will be just so much more carbon to form either graphite or cementite. The amount of graphite will depend upon several conditions. A hot blast 'furnace will give a higher percentage than a cold furnace, and high silicon will also cause the separation of free carbon, while manganese and sulphur will cause the carbon to remain combined. Cast-iron with 1.25 per cent, combined carbon is really steel, but weakened and embrittled by graphite. In the same way cast-iron with 3 per cent, of combined carbon plus 1 per cent, of graphite is a mechanical mixture of two substances: (1) 99 parts white cast-iron containing 3 per cent, of combined carbon, and (2) 1 part of graphite. The contention that graphite "weakens and embrittles" cast-iron is founded on the fact that pig-irons containing the same proportion of silicon., manganese and sulphur carry the same proportion of total carbon, no matter whether they are gray or white. An increase in graphite means a decrease in combined carbon, and since one-quarter of the carbon is in the form of pearlite, and since cementite must contain 6.57 per cent, of carbon, it follows that if much carbon exists as graphite, the proportion of cementite decreases and the proportion of soft ferrite increases, with a toughening of the mass. This toughening is usually ascribed to graphite, •when in reality the graphite weakens the iron by destroying its. continuity. Thus silicon will toughen iron because it drives the carbon into the condition of graphite, while manganese will make, it brittle because it causes it to combine.